1. Technical Field
The present invention is directed to the art of RF broadcast transmission systems and, more particularly, to improving the linearity of an inductive output tube (IOT).
2. Description of the Prior Art
It is known that an inductive output tube (IOT) has particular application for use in television broadcasting wherein high kilowatt level RF power is required. Examples of an IOT include the U.S. Patents to Preist, et al. U.S. Pat. No. 4,480,210, Symons U.S. Pat. No. 5,650,751, Pickering, et al. U.S. Pat. No. 5,691,667 and Bardell U.S. Pat. No. 5,736,820 the disclosures of which are herein incorporated by reference.
Inductive Output Tubes or IOT, as they are commonly called, are high vacuum electron tubes, which allow an electron beam to travel from one end to another in a controlled way. There are four primary parts to an IOT: a cathode which emits electrons, an anode which accelerates the electrons, a collector which collects the electrons, and a grid for controlling the electron emission. The electrons are emitted from a spherical surface cathode consisting of a tungsten matrix heated from behind by a tungsten heater. A spherical pyrolytic carbon grid is positioned close to the cathode and controls the emissions of electrons from the cathode. The cathode is maintained at a relatively high potential (-35,000 volts for typical tubes) while the grid is at a relatively low potential (-50 to -250 volts for typical tubes) with respect to the cathode. If the grid is made less negative with respect to the cathode, then more electrons are emitted. The high electric field between the cathode and anode makes the emitted electrons travel toward the anode or collector. A magnetic field is used to focus the electrons into a beam. Emitted electrons are collected in the collector completing the circuit.
For use as an amplifying device, a radio frequency cavity is positioned such that it can induce a voltage between the grid and cathode, thus modulating the electron emission from the cathode. The electrons emitted from the cathode are accelerated as they travel toward the anode. If a second radio frequency cavity is placed between the cathode and anode in such a way that the electron beam passes through the cavity, then the electrons passing through the cavity will induce an RF voltage in the cavity. This RF voltage can then be coupled from the cavity. It should be noted that the tuned cavity and electron beam form the complete resonant circuit. Changes in the electron beam can shift the resonant frequency of the cavities.
The Inductive Output Tube is used primarily as a high power UHF amplifier. One primary use is in UHF television transmitters operating in the frequency range of 470 MHz to 860 MHz. It is used both for analog television and digital television transmissions. In order to obtain good efficiency, the IOT is operated in a class A/B mode of operation. Due to the class A/B mode of operation, the amplifier draws current which is proportional to the modulation frequencies of the RF signal applied. For analog and digital television signals, these modulation frequencies cover the range of DC through 8 MHz and are commonly called video currents.
In the construction of an IOT, the pyrolytic grid is extremely fragile. Due to the high acceleration voltages used, it is possible for the tube to arc from grid to anode. If an arc occurs, the high tension supply may destroy the grid. To overcome this problem, a crowbar or other current limiting device is placed between the IOT and the high tension supply. If an arc occurs, the crowbar directs the high tension supply current away from the IOT preventing the delicate grid from being damaged. Common crowbars use either a gas filled thyratron or a triggered spark gap. These crowbars use a controlled arc to divert the current from the high tension supply away from the IOT. Since the undesired arc in the IOT and the controlled arc in the crowbar have the same impedance, a series resistor must be placed between the crowbar and the IOT, thus forcing the high tension current through the crowbar and away from the IOT.
Since the IOT draws video currents from the high tension supply, the series resistance causes a voltage drop. This voltage drop has the undesirable effect of modulating the cathode voltage. This modulation of the cathode voltage causes an undesirable effect of re-modulation. Re-modulation is due to the fact that a change in the cathode voltage produces a corresponding change in the amplifier's gain and phase characteristic. These gain and phase changes are the cause of AM to AM and AM to PM distortions and are also referred to as non memory-full distortions (non-linear distortions). Also, any changes to the cathode voltage causes a corresponding change in the current density of the electron beam. As the electron beam passes through the cavities, the density changes have the effect of changing the frequency response or tuning of the cavities. This dynamic change in frequency response also causes the undesirable distortion called memory-full distortion (linear distortion).
To further compound the problem, the high tension supply is typically located between several to 100 feet from the amplifier. The inductance of the interconnect wire at these lengths to video currents also introduces a voltage drop to the cathode voltage at the tube which further compounds the effects of re-modulation.
The emission of electrons from the cathode is greatly enhanced when the cathode is operated at elevated temperatures. In practice, this is accomplished by heating the cathode with a filament. A typical IOT filament draws between 10 and 30 amperes of current. To prevent the filament from emitting electrons, it is embedded within the cathode to shield it from the high tension acceleration voltage. To prevent the filament from acting as an anode, one end is connected to the cathode. Thus, the cathode lead to the tube must not only supply the video current due to the modulated radio frequency signal applied but it must supply the filament current simultaneously.
In the construction of an IOT, ceramic materials are used to support the cathode, grid, anode and collector structures. These ceramic materials create small capacitors which exists between the cathode and anode (typically 1000 pF) and grid to anode (typically 100 pF).
The radio frequency input cavity connects between the cathode and the grid. This cavity must also provide isolation from the high tension supply. The insulating materials used in its construction also generate capacitors from cathode to anode and grid to anode.
When the cathode is heated by a filament, electrons in the cathode are emitted but cannot travel to the collector due to the control grid blocking their path. A cloud of electrons forms between the cathode and grid. This cloud of electrons forms a capacitor.
Since the cathode has mass and is heated, there is stored energy in the cathode. This stored energy frees electrons for emission but they cannot be emitted since the area between the cathode and control grid is already filled with electrons. These freed electrons in the cathode also create a capacitance.
This invention makes use of these capacitors to create an effective video bypass for the high tension supply. By placing an inductor in series with the cathode, an effective low pass filter is created from the above capacitors and the added inductor. This filter is highly effective for the video frequencies. This filter has the effect of reducing the cathode ripple due to the series resistance and long wire inductance to the high tension supply. Video currents are supplied from these capacitors and the high tension supply must now only provide only the average current. Voltage drop at the cathode is reduced and the undesired re-modulation effects are also reduced.
A further improvement can be obtained by the addition of capacitor placed across the grid supply. This capacitor effectively couples the grid to anode capacitance to the cathode providing further reduction to the high tension supply ripple.